The present invention relates to a readhead for a photometric diagnostic instrument, such as a reflectance spectrometer, for performing tests on a sample of body fluid to be analyzed.
It is useful for various medical diagnostic purposes to utilize a reflectance spectrometer to analyze samples of body fluid, for example, to detect the blood glucose level of infants. Conventional reflectance spectrometers have been used to detect the presence of glucose in a blood sample disposed on a reagent pad. Glucose present in the blood reacts with the reagent on the reagent pad, causing the reagent pad to change color to an extent which depends on the glucose concentration in the blood.
A readhead for use in a spectrometer device is disclosed in U.S. Pat. Nos. 5,518,689 and 5,611,999 to Andrew Dosmann, et al. That readhead is composed of a housing, a pair of light-emitting diodes fixed relative to the housing and adapted to emit light towards a target area in which a reagent pad is located, and a sensor for detecting light emitted from the illuminated reagent pad.
The accuracy of reagent tests of the type described above is dependent on the sensitivity of the reflectance spectrometer to non-uniform color distributions in the reagent pad. It is desirable that the spectrometer is relatively insensitive to such non-uniform color distributions. It has been realized that two important optical characteristics that minimize inaccuracies caused by non-uniform color development are the size of the reagent area that is read by the instrument and the uniformity of the illumination of the reagent pad.
Some prior art techniques used to attempt to improve the quality of an illumination system include the use of ground glass or frosted Mylar plastic in front of the light source. However, those techniques were tested and found to be unacceptable due to excessive expansion of the light beam, relatively large light output loss, and increased stray light.